National Aeronautics and Space Administration GDC Orbit Primer Michael Mesarch [email protected] October 10, 2018 c o d e 5 9 5 NAVIGATION & MISSION DESIGN BRANCH NASA GSFC www.nasa.gov Outline Orbital Elements Orbital Precession Differential Rates String of Pearls In-Plane Drift ΔV Calculations Orbital Decay Radiation Dosage NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 2 Earth Orbital Elements Six elements are needed to specify an orbit state Semi-major Axis (SMA, a) Half the long-axis of the ellipse Perigee Radius (Rp) – closest approach to Earth Apogee Radius (Ra) – furthest distance from Earth SMA = (Rp + Ra)/2 Eccentricity (e) – ellipticity of the orbit e = 0 Circle 0 < e < 1 Ellipse e = 1 Parabola e = Ra/a -1 or e = 1 – Rp/a Inclination (i) Angle orbit makes to Earth equator Argument of Periapsis (ω) – angle in Angle between orbit normal vector and North Pole orbit plane from ascending node to Right Ascension of Ascending Node (Ω) perigee Angle in equatorial plane between Ascending Node True Anomaly (TA) – angle in orbit and Vernal Equinox plane from perigee to orbital location NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 3 Nodal Precession (Regression) The Earth’s equatorial bulge imparts an out-of-plane force on the orbit which causes a gyroscopic precession of the line of nodes 2 푑Ω −3푛퐽2푅퐸푐표푠푖 rad/sec 푑푡 2푎2(1−푒2)2 where Polar orbits have no nodal precession R – Earth Radius When nodal precession is equal to the E apparent motion of the Sun (0.9856 n – mean motion ( 휇/푎3) deg/day … 360 deg in 365.2422 days) J2 – Geopotential term related to then we have a Sun-Synchronous orbit Earth’s oblateness NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 4 Nodal Precession - 80°Inclination NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 5 Nodal Precession - 83°Inclination NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 6 Nodal Precession – 700 km, Circular For a 700 km, circular orbit, the Sun-synchronous inclination is 98.2 Sun-Synchronous Rate deg NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 7 Apsidal Precession Similar to nodal regression, the Earth’s oblateness causes a precession of the orbital line of apsides 푑휔 3푛퐽 푅2(4−5푠푖푛2푖) = 2 퐸 rad/sec 푑푡 4푎2(1−푒2)2 where RE – Earth Radius n – mean motion ( 휇/푎3) Perigee and apogee will precess J – Geopotential term related to Earth’s 2 in latitude over time oblateness NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 8 Apsidal Precession - 80°Inclination NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 9 Apsidal Precession - 83°Inclination NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 10 Differential Precession A mission designer can use differential precession to space orbit planes apart from each other This can be achieved by maneuvering 1 satellite into a different orbit by changing apogee or perigee and allowing it to drift relative to the other satellite(s) However, the precession is in the Ascending Node – the inclination remains the same Fuel costs for inclination changes are incredibly high NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 11 Differential Precession Example In this example, we can see the differential nodal drift relative to a 450 km circular orbit with an inclination of 80° NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 12 Differential Precession Movie NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 13 Nodal/Apsidal Interactions The differences in the nodal and apsidal precession rates can create interesting observation opportunities The perigee latitude changes as the argument of perigee (ω) precesses The perigee local time changes as the ascending node (Ω) precesses. Furthermore, the local time will ‘flip’ by 12 hours as perigee passes over the pole An example of this feature is shown on the following slides for a 200 x 2000 km orbit with varying inclination, simulated 2 years The plots show the Latitude and Local Solar Time of the perigee position NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 14 Precession Movie NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 15 Perigee Latitude & Local Time vs. Time Over this inclination range (75 – 80 deg) Absolute apsidal rates increase as inclination increases Absolute nodal rate decrease as inclination increases NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 16 Perigee Latitude vs Local Time Judicious selection of orbit parameters and inclination can yield interesting patterns This example has a “Frozen Orbit” at an inclination near 77.5 80, and 83.5 deg NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 17 String of Pearls Launching multiple spacecraft and spacing them out in a “string of pearls” is a simple operation A small maneuver(s) can be performed to change the orbital period slightly to allow for “in-plane” drift Once the desired separation has been achieved then maneuver(s) can be performed to stop the drift Ultimately, the cost of the drift maneuvers is dependent on how long is allowable to achieve the desired separation NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 18 In-Plane Drift As requested, the ΔV cost vs. time to drift was computed for a 500 km orbit The quicker the drift, the more ΔV is necessary NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 19 Plane Change ΔV Plane changes are proportional to the Nodal rotations are slightly more orbital velocity complicated as the orbital plane change is a function of both the Δ훼 inclination and the nodal change Δ푉 = 2푉푖sin( ) 2 (using spherical trigonometry) For a satellite flying in a 500 km orbit 푐표푠훼 = 푐표푠푖푖푐표푠푖푓 + 푠푖푛푖푖푠푖푛푖푓 cos ΔΩ (Vcirc = 7.613 km/s) the cost to change inclination by 1 deg is 133 ii & if – initial/final inclination m/s ΔΩ – nodal change Using the previous example, a 30 deg nodal rotation in a 80 deg inclination orbit (without changing inclination) costs 3881 m/s!!! Plane changes in low-Earth orbit are expensive NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 20 ΔV & Propellant Calculations Propellant mass fraction is a function of ΔV and propellant specific impulse (Isp) 푀푃 = 1 − 푒−Δ푉/푔퐼푠푝 푀푖 Common Isp values Hydrazine: 220 sec BiPropellant: 310 sec NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 21 Orbital Decay Orbital decay is a function of Solar Activity and spacecraft ballistic properties Solar activity is obtained via solar flux predictions The spacecraft ballistic properties are a function of the Mass/Area ratio The larger Mass/Area ratio, the more resistant the spacecraft is to atmospheric drag, the slower the orbital decay Need to take into account all appendages (e.g. solar arrays, booms, appendages) as well as the attitude profile when computing the drag area As mass is expended (from thruster firings) the spacecraft becomes less resistant to atmospheric drag Because of these variables, it’s difficult to generalize on lifetime durations NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 22 Solar Flux Prediction NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 23 Orbit Decay (days) from 450 to 425 km Decay times range from 3 months to 3 years NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 Radiation Dosage David Batchelor (Code 586) computed the Total Ionizing Dose (TID) for the orbit regimes that were requested and the results are below Orbit 300 km 700 km 300 x 1200 km TID (krad) 2.4 6.0 11.9 Assumptions Shield Thickness: 100 mil Al Orbit Inclination: 82 deg Lifetime: 1 year Launch Date: 1/1/2023 (Solar Max) Solar minimum conditions reduces TID by 38%, 17%, and 8%, respectively NAVIGATION & MISSION DESIGN BRANCH, CODE 595 GDC Orbit Primer NASA GSFC October 10, 2018 25.